Uptake of dissolved inorganic nitrogen by shallow seagrass communities exposed to wave-driven unsteady flow

Abstract

In natural seagrass systems, regular oscillatory motion -like that caused by surface wind-waves -enhances uptake of dissolved inorganic nitrogen (DIN) relative to current-driven, unidirectional flows. A mobile field flume was deployed to measure the uptake of DIN by intact shallow seagrass communities exposed to unidirectional, oscillatory, and combined flows. The flume volume was spiked with 15 N-labeled DIN to measure nutrient uptake rate constants for the entire system (S) and to determine height-specific nutrient uptake rates of seagrass epiphytes (ρ). In oscillatory and combined flows, S depended positively on the water speed, but also inversely on a modified Keulegan-Carpenter number (KC). This ratio compares the wave-orbital excursion distance to canopy element spacing. Experiments characterized by relatively low KC values were associated with significantly higher uptake efficiencies than experiments characterized by high KC values. Uptake efficiencies for high KC conditions were similar to those measured in comparable unidirectional flows. Measured canopy flow attenuation was also found to increase with KC, a result in line with expectations from a parameterized conceptual model. Damping variability alone could not explain the observed oscillatory flow uptake enhancement however, a result that seemingly highlights the role of both element flexural response and unsteady boundary layer mechanics in the exchange process. In all flow conditions, DIN uptake rates of epiphytes harvested from the bottom of the blades (ρ bot ) were lower than those for epiphytes harvested from the upper portion of the blades (ρ top ). While uptake by the top epiphytes exhibited flow dependency, uptake by bottom epiphytes did not.

A simple and practical model for combined wave–current canopy flows

“…Wave-driven flows are common in coastal ecosystems and their dynamics differ significantly from the dynamics of unidirectional flow (Lowe, Koseff & Monismith 2005). Wave forcing is often the dominant driver of processes of interest in coastal systems, such as sediment resuspension, nutrient uptake and biomechanical failure (Storlazzi et al 2005;Hansen & Reidenbach 2011;Weitzman et al 2013). Thus, practical models for wave-driven canopy flow are required for a variety of applications, from ecosystem conservation to coastal protection.…”

“…The terrestrial canopy flow literature provided the initial framework for studying aquatic canopies and similarities have been found over a range of scales and applications (Ghisalberti 2009). More recently, oscillatory canopy flow has been a growing area of research (Lowe et al 2008;Bradley & Houser 2009;Luhar et al 2010;Pujol & Nepf 2012;Weitzman et al 2013). Wave-driven flows are common in coastal ecosystems and their dynamics differ significantly from the dynamics of unidirectional flow (Lowe, Koseff & Monismith 2005).…”

Effects of Oscillatory Flow on Fertilization in the Green Sea Urchin Strongylocentrotus droebachiensis

“…The signal was ensemble averaged using period as the fundamental length. Then rms( u w ) was estimated as the root-mean-square of the resulting ensembled wave [22].…”

Exploring coral reef responses to millennial-scale climatic forcings: insights from the 1-D numerical tool pyReef-Core v1.0

“…High water flow increases rates of photosynthesis by symbiotic algae (Bruno and Edmunds, 1998), nutrient uptake by corals (Weitzman et al, 2013) and particle capture (Houlbrèque and Ferrier-Pagès, 2009) and facilitates sediment removal from coral surfaces (Rogers, 1990), all of which contribute to enhanced primary production. At the extremes, too little flow can be lethal in corals by inducing anaerobiosis, whereas extreme wave events cause mechanical destruction (Done, 2011) and can lead to long-term changes in community diversity and structure (Madin and Connolly, 2006).…”